1. Field of the Invention
The current invention relates to a corrosion resistant coating of rollers for use in paper machines.
2. Description of the Related Art
During the production of paper which, in this document, is to be understood to include all cellulose containing fibrous webs, for example cardboard, nonwoven fabrics and similar, the rolls of the paper machine are subjected to process water and steam with electrolytic conductivity. In order to prevent these aggressive mediums reaching the roll cores manufactured from base metals the rolls, or respectively the roll cores, are coated with a corrosion resistant material.
As a corrosion protective coating for rolls, or respectively roll cores, galvanic hard chrome layers and hard metal layers applied by means of thermal spray coating is used. The galvanic chrome plating provides an effective corrosion protection which can be produced cost effectively. However due to toxicity of the used electroplating baths, this must be categorized as being an environmentally harmful method. Moreover, mechanical cleaning of galvanic hard chrome layers is difficult due to the brittle characteristics of chrome. A composite material of tungsten carbide hard materials embedded in a cobalt-nickel-matrix is normally used as hard metal coating layers which are applied to the surface by means of a thermal spray process, for example HVOF (high velocity oxygen fuel, high velocity flame spraying). The application of thermal hard metal spray layers occurs in several cycles, that is in several partial layers, in order to ensure the physical homogeneity of the coating.
Hard metal coatings applied in a thermal spraying process are characterized by effective corrosion protection and high abrasion resistance. Adhesion of contaminants occurring during paper production is comparatively low on hard metal surfaces, so that these can be cleaned relatively easily through doctoring. However, the production of thermal hard metal coating compared to chrome plating is very cost intensive.
At less critical locations in a paper machine, for example guide rolls for fabrics or felts, where the corrosion protective coating does not come into contact with the paper surface, the more cost-effective galvanic hard chrome plating is therefore currently used which is, however, linked to greater environmental pollution. This type of hard chrome plating can however not be doctored.
What is needed in the art is a cost effective corrosion protection which, compared to galvanic hard chrome plating, is less harmful to the environment but nevertheless effective for roll components.